1. Field of the Invention
The present invention relates to immersion lithography, more particularly, to providing side seals for a wet lens element to protect the wet lens element from degradation.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays (e.g., liquid crystal displays), semiconductor wafers, circuit boards, various integrated circuits, print heads, macro/nano-fluidic substrates, and the like. During lithography, a substrate, which is disposed on a substrate stage, is exposed to an image projected onto the surface of the substrate by exposure optics located within a lithography apparatus.
The projected image produces changes in the characteristics of a layer, for example, photoresist, deposited on the surface of the substrate. These changes correspond to the features projected onto the substrate during exposure. Subsequent to exposure, the layer can be etched or otherwise processed to produce a patterned layer. The pattern corresponds to those features projected onto the substrate during exposure. The patterned layer is then used to remove or further process exposed portions of underlying structural layers within the substrate, such as conductive, semiconductive, or insulative layers. This process is repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the substrate.
In the field of immersion lithography the exposure operation is conducted with an immersion liquid, which is typically water, between the last lens element of the projection lens and the substrate. This requires certain design modifications to the apparatus. A number of such lithographic apparatus use calcium fluoride (CaF2) lenses as the last element in the projection lens. This element is therefore constantly in contact with the immersion liquid. To achieve the required optical properties such immersion lithography apparatus often use ultra pure water (“UPW”). Unfortunately, calcium fluoride dissolves in ultra pure water and the lens needs to be protected from the water in order to prevent surface damage that can lead to unwanted scattering of light. For example, when calcium fluoride dissolves into the immersion liquid, some of the calcium fluoride will crystallize onto a work surface, such as a wafer, from microdroplets that are left behind. These microdroplets can lead to imperfections in the printed image on a wafer or other work surface.
Non-optical areas of the lens also dissolve calcium flouride into the water. Due to the water conditions, the rate of dissolution far exceeds the purity specifications of the water. Consequently, the purity of the water (or other liquid) is affected which leads to defects to wafers or other surfaces. It is therefore necessary to protect both the optical surfaces of a lens and the non-optical sides of the lens that come in contact with the water.
What is needed is a method for protecting the non-optical sides of a wet lens element from degradation which are immersed in a liquid.